The present invention relates to a secondary battery case having excellent mechanical strength, impact resistance, heat resistance, chemical resistance and high weld strength of a weld which have occurred in a molding process, as well as an adhesive strength of welding one parts with at least one of other parts, which battery case can be used in the fields of electrical and electronic devices, automobiles, and various other industrial products.
Closed secondary batteries, such as lead storage battery, nickel-cadmium battery and nickel-hydrogen battery, are widely used as power source of vehicles, especially oversized or special vehicles, electric apparatus, and other various kinds of industrial equipment, and their demand has steadily increased in recent years. According to these demands, enlargement of battery case, diversification of its design, reduction of weight and thickness, etc., are required. Therefore, as regards the material used therefor, request for its further improved qualities such as better moldability, higher strength, higher heat resistance and improved vapor barrier properties has become intense.
As resin for storage battery case, ABS resin (acrylonitrile-butadiene-styrene copolymer) has been used predominantly. ABS resin excels in moldability and also has a good balance of rigidity, strength and dimensional precision, and in fact, it has realized many glorious results in practical use for battery case. ABS resin, however, may prove unsatisfactory in heat resistance or strength in certain uses, and in such cases, modified polyphenylene ether resin has been used. This modified polyphenylene ether resin is a mixture of polyphenylene ether and polystyrene, and it can be provided with desired heat resistance by property changing the ratios of said materials. Also, it is relatively easy to afford flame retardancy to the said resin, so that the said resin is suited for applications where flame retardancy is required.
Both of these resins, however, are permeable to water vapor and have the disadvantage of being poor in vapor barrier properties. So, in case where these resins are used for battery case, water in the electrolyte contained in the battery, which evaporates in long-time use, is allowed to pass through the battery case and dispersed into the atmosphere. Use environment of the batteries tends to become higher in temperature and lower in humidity due to elevation of output and other factors, which serves for elongating the expected service life whereby the electrolyte of the battery tends to reduce. Decrease of electrolyte leads to enlargement of internal resistance of the battery to greatly interfere with discharge performance of the battery, so that development and use of a resin which is as much impervious to water vapor as possible has been desired. ABS resin and modified polyphenylene ether resin, as they are both amorphous type resins, are poor in chemical resistance and therefore unsuited for use as material of the parts which have a possibility to come into contact with chemical substances such as oil in use.
On the other hand, use of polypropylene resin is advantageous in terms of water-vapor barrier effect and chemical resistance. Polypropylene resin is indeed excellent in water vapor barrier properties, chemical resistance and moldability, but it is not necessarily satisfactory in mechanical strength and heat resistance and lacks in long-term reliability as it is poor in creep resistance.
From these circumstances, a closed type secondary battery case made of a resin composition comprising a crystalline polyolefin resin and a polyphenylene ether-based resin was proposed in Japanese Patent Application Laid-Open (KOKAI) No. 8-195188, and a closed type alkali battery using a polymer alloy comprising principally a polyphenylene ether resin and a polyolefin resin was disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 9-120801.
However, the closed secondary battery cases made of these resin compositions, although improved in water-vapor barrier properties, etc., to a certain extent, are still unsatisfactory in weld strength (it is often called as xe2x80x9cknit strengthxe2x80x9d) in case where the molded product used as battery case has a weld which have occurred in a molding process. The weld of a molded product occurs in the area where the flows of molten resin in the mold join each other in the molding operation, and is usually lower in the weld strength than other portion. Therefore, low weld strength may cause fracture of the molded product from its weld zone, making it unable to obtain the expected performance of the molded product.
Also, in manufacturing a closed secondary battery, there are the occasions where two molded parts, such as the cover and the body (container) need to be welded with each other. In such occasions, the conventional blend of polyphenylene ether-based resin and polypropylene resin may fail to provide sufficient bonding strength, and when a strong internal pressure is exerted to the battery, the adhesive zone may be unable to endure and may give way. Thus, the problem of low adhesive strength of welding has been raised in the art of manufacture of secondary battery case.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that a secondary battery case comprising a resin composition prepared by blending a polyphenylene ether-based resin, a crystalline polypropylene resin having specific properties and a block copolymer in the specific ratios is provided with excellent mechanical strength, heat resistance, impact resistance, chemical resistance and water-vapor barrier properties as well as high weld strength and adhesive strength of welding.
In the present invention, xe2x80x9cweld strengthxe2x80x9d means a strength at the weld portion (zone) which have occurred in a molding process, and it is often called as xe2x80x9cknit strengthxe2x80x9d. And xe2x80x9cadhesive strength of weldingxe2x80x9d means a strength at the adhesive portion (zone) which is generated when the products is made by two or more (plural) parts molded by the thermoplastic composition, and one of the plural parts and at least one of other parts are welded with each other to adhere.
The present invention has been attained on the basis of the above finding.
The object of the present invention is to provide a secondary battery case having excellent mechanical strength, heat resistance, impact resistance, chemical resistance and water-vapor barrier properties as well as high weld strength and adhesive strength of welding.
To attain the above aim, in the first aspect of the invention, there is provided a secondary battery case being molded by a thermoplastic resin composition comprising:
(A) a polyphenylene ether-based resin,
(B) a crystalline polypropylene resin which is a homopolymer or a copolymer of polypropylene, and
(C) a hydrogenated block copolymer comprising an aromatic vinyl compound polymer block and a conjugated diene compound polymer block,
the (A)/(B) ratio by weight being 7/93 to 70/30,
the amount of (C) being 2 to 40 parts by weight based on 100 parts by weight of (A) and (B) combined,
the melt flow rate of said crystalline polypropylene resin (B) being 0.1 to 10 g/10 min as measured according to JIS K-7210 at 230xc2x0 C. and 21.17 N,
the density of the propylene polymer moiety in said crystalline polypropylene resin being not less than 0.906 g/cm3.
In the second aspect of the present invention, there is provided a secondary battery case being molded by a thermoplastic resin composition comprising:
(A) a polyphenylene ether-based resin,
(B) a crystalline polypropylene resin which is a homopolymer or a copolymer of polypropylene, and
(C) a hydrogenated aromatic vinyl compound-conjugated diene block copolymer containing 50 to 85% by weight of aromatic vinyl compound units,
the (A)/(B) ratio by weight being 7/93 to 25/75,
the amount of (C) being 3 to 30 parts by weight based on 100 parts by weight of (A) and (B) combined,
the melt flow rate of said crystalline polypropylene resin (B) being 0.1 to 10 g/10 min as measured according to JIS K-7210 at 230xc2x0 C. and 21.17 N,
the density of the propylene polymer moiety in said crystalline polypropylene resin being not less than 0.906 g/cm3, and
the ratio by weight of (A) to (C), (A)/(C), being 1/1 to 10/1.
In the third aspect of the present invention, there is provided a secondary battery case according to the first aspect, which is a molded product of the thermoplastic resin composition and has a weld zone which have occurred in a molding process.
In the fourth aspect of the present invention, there is provided a secondary battery case according to the first aspect, which contains two or more molded products of the thermoplastic resin composition, the said molded products being welded with each other.